126 ANNULOIDA. 



alternate with one another, and which are termed "inter- 

 radials " (/). Lastly, one of the interradial spaces, correspond- 

 ing with the anus, is usually much wider than the others, and 

 is furnished with an additional series of plates, which are called 

 the "anal plates" (a). 



As regards their general distribution in time, the Crinoidea 

 present us with an excellent example of a group which early 

 attained its maximum of development, and which has now 

 dwindled down to some half-dozen surviving species. With 

 one or two doubtful exceptions, the Crinoids appear, so far as 

 yet known, to have commenced their existence in the Lower 

 Silurian period, and they are represented by numerous and 

 very varied forms in the seas of the Upper Silurian period. 

 Amongst the commoner Silurian genera may be mentioned Cro- 

 talocrinus, Glyptocrinus, Ichthyocrinus, Periechocrinus, Rhodo- 

 crinus, Poteriocrinus, and Taxocrinus. In the Devonian Rocks, 

 also, Crinoids are plentiful, and the genera Cupressocrinus, Hap- 

 locrinus, and Melocrinus, may be mentioned as peculiar to this 

 period. It is in the earlier portion, however, of the Carbonifer- 

 ous period that the Crinoids attain their highest development. 

 The Carboniferous Limestone is in many places, over wide 

 areas, and for a thickness of many yards, almost entirely made 

 up of the debris of Crinoids; and in many places it is so 

 charged with the remains of these organisms as to deserve and 

 acquire the name of " crinoidal limestone " or " entrochal 

 marble." Amongst the commoner Carboniferous genera may 

 be mentioned Cyathocrinus, Actinocrinus (fig. 74), Platycrinus 

 (fig. 73), Poteriocrinus, and Taxocrinus. It is in the Palaeozoic 

 period, then, that the Crinoids attain their maximum, both 

 numerically and as regards the number of genera and species. 

 Taken as a whole, the Palaeozoic Crinoids are distinguished 

 by the characters already mentioned ; namely, by having the 

 brachial grooves conveyed to the mouth in concealed channels 



j or tunnels, in having mostly a rounded column, and in the 

 fact that the articular facets of the column-joints usually are 

 marked by numerous simple radiating striae. As a rule, also, 

 the Palaeocrinoids have a calyx, in which the dorsal surface has 



~ a preponderating development as compared with the ventral 

 surface. 



Coming to Mesozoic times, Crinoids are still abundant, 

 though certainly reduced in numbers as compared with their 

 development in the Palaeozoic period. Taken as a whole, the 

 Mesozoic Crinoids are characterised by having the brachial 



1 grooves continued over the ventral surface of the disc, and by 



; having the ventral portion of the calyx more extensively de- 



